The present invention relates to eccentric drilling tools for combined rotary and percussive drilling in earth and in rock covered by overburden concurrently with a casing tube following the drilling tool downhole, and more particularly to the type of such drilling tools wherein a guide member is rotatably centered by the mouth of the casing tube, a pilot bit at the forward end of said tool is supported by the guide member and provided with axially oriented hard metal pilot insert means peripherally thereon directed to define the diameter of the pilot hole to be cut during drilling, a laterally extending eccentric bit behind said pilot bit on the tool carries axially oriented hard metal reamer insert means at the eccentric top thereof directed to ream up the pilot hole to the maximum diameter of the hole drilled, and the eccentric bit is movably supported relative to said guide member between a drilling position, in which the eccentric bit projects laterally beyond said casing tube so as to drill jointly with the pilot bit a hole larger than the casing tube, and a retracted position, in which the drilling tool can be retracted or lowered through the casing tube.
Drilling tools of this type are disclosed inter alia in U.S. Pat. publications 3,753,470, 3,848,683, and 4,440,244. In these tools the greater part of the drilling work falls on the insert means that have the longest distance to travel during work, i.e. on the reamer insert means at or close to the maximum diameter worked. In that region primarily the insert means leading in the rotational direction of the tool is the one that has to, or in case of radially staggered plural insert means, are the ones that have to carry the maximum load since being the first to "break up" the hole radially to its full dimension Under the action of the rotary drive torque and the vibrations during drilling, the load on these leading inserts tends to turn the eccentric bit in the rotational drilling direction with the leading insert means as central point. As a result there is created an undesirable wear of the pilot bit towards a gradual reduction of the maximum diameter drilled until finally the casing tube gets stuck.